High altitude platforms (aircraft and lighter than air structures situated from 10 to 35 km altitude)—HAPS, have been proposed to support a wide variety of applications. Areas of growing interest are for telecommunications, positioning, observation and other information services, and specifically the provision of high speed Internet, e-mail, telephony, televisual services, games, video on demand, and global positioning.
High altitude platforms possess several advantages over satellites as a result of operating much closer to the earth's surface, at typically around 20 km altitude. Geostationary satellites are situated at around 40,000 km altitude, and low earth orbit satellites are usually at around 600 km to 3000 km altitude. Satellites exist at lower altitudes but their lifetime is very limited with consequent economic impact.
The relative nearness of high altitude platforms compared to satellites results in a much shorter time for signals to be transmitted from a source and for a reply to be received—which has an impact on the “latency” of the system. Moreover, high altitude platforms are within the transmission range for standard mobile phones for signal power and signal latency. Any satellite is out of range for normal mobile phone handsets operating without especially large or specialist antennas.
High altitude platforms also avoid the rocket propelled launches needed for satellites, with their high acceleration and vibration, as well as high launch failure rates with attendant impact on satellite cost.
Payloads on high altitude platforms can be recovered easily and at modest cost compared to satellite payloads. Shorter development times and lower costs result from less demanding testing requirements.
U.S. Pat. No. 7,046,934 discloses a high altitude balloon for delivering information services in conjunction with a satellite.
US 20040118969 A1, WO 2005084156 A2, U.S. Pat. No. 5,518,205 A, US 2014/0252156 A1, disclose particular designs of high altitude aircraft.
However, there are numerous and significant technical challenges to providing reliable information services from high altitude platforms. Reliability, coverage and data capacity per unit ground area are critical performance criteria for mobile phones, device communication systems, earth observation and positioning services.
Government regulators usually define the frequencies and bandwidth for use by systems transmitting electromagnetic radiation. The shorter the wavelength, the greater the data rates possible for a given fractional bandwidth, but the greater the attenuation through obstructions such as rain or walls, and the more limited diffraction which can be used to provide good coverage. These constraints result in the choice of carrier frequencies of between 0.7 and 5 GHz in most parts of the world with typically a 10 to 200 MHz bandwidth.
There is a demand for high data rates per unit ground area, which is rapidly increasing from the current levels of the order 1-10 Mbps/square kilometre.
To provide high data rates per unit ground area, high altitude unmanned long endurance (HALE) aircraft, or free-flying or tethered aerostats, need to carry large antenna(s) to distinguish between closely based transceivers on the ground. A larger diameter antenna leads to a smaller angular resolution of the system, hence the shorter the distance on the ground that the system can resolve. Ultimately the resolution is determined by the “Rayleigh criterion” well known to those skilled in the art. The greater the antenna resolution, the higher the potential data rates per unit ground area are.
However fitting extremely large diameter antenna or antennas of 50 metres or more onto platforms is not feasible with current or envisaged platform technology.
Synthetic aperture synthesis has long been known in radio astronomy where the resolution equivalent to that of a large antenna is synthesized by using the signals from appropriately spaced relatively small antennas. Indeed Martin Ryle and Antony Hewish shared the Nobel prize for physics in 1974 for this and other contributions to the development and use of radio interferometry. Related technology to aperture synthesis has been used for many years in low frequency radio communication to submarines, in acoustics, and phased arrays in LTE and WiFi systems. However its use for communication to and from ground based or aerial user equipment has never been previously considered.
Low frequency radio communication to submarines and the use of acoustic synthetic aperture synthesis have been used for some years.
Phased array digital “beamforming” (DBF) and multi beam horn (MBH) antennas have been considered for high altitude platforms in for example, R. Miura and M. Suzuki, “Preliminary Flight Test Program on Telecom and Broadcasting Using High Altitude Platform Stations,” Wireless Pers. Commun., An Intl. J., Kluwer Academic Publishers, vol. 24, no. 2, January 2003, pp. 341-61. Other references include:
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=620534,
http://ieeexplore.ieee.org/xpls/abs_all.jsp?arnumber=933305,
http://digital-library.theiet.org/content/journals/10.1049/ecej_20010304,
http://digital-library.theiet.org/content/journals/10.1049/el_20001316
http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=4275149&pageNumber %3D129861
However, the prior art suggest that use of high altitude platforms does not represent a promising way forward to delivering next generation communication means.